Speculative fix for mac build failure.

R=thakis@chromium.org
BUG=585261

Review URL: https://codereview.chromium.org/1679843003 .
diff --git a/build/standalone.gypi b/build/standalone.gypi
index 72975bc..4c674ed 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -431,6 +431,7 @@
     ],
   },
   'xcode_settings': {
+    'SDKROOT': 'macosx10.10'  # -isysroot
     # See comment in Chromium's common.gypi for why this is needed.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },